Induction Motors - Electrical Technology -

1. Chapter # 34
INDUCTION MOTORS

2. Induction Motor: General Principle
As generally conversion of electrical -> mechanical is in the rotatory part of the machine
In DC motors the electric power is conducted directly to the armature through brushes & commutator so because of this
we called the DC motors as Conduction Motors
But in AC motors the rotator doesn’t receive electric power by conduction but it is done by induction in exactly the same
way as the secondary of a 2-winding transformer receive its power from the primary. So we call such motors as
Induction Motors
In fact the induction motor can be treated as a Rotatory transformer i.e one in which the primary winding is stationary
but the secondary is free to move
INDUCTION MOTOR (ROTATORY TRANSFORMER)
ADVANTAGES DISADVANTAGES
Very simple, rugged, almost unbreakable & low cost Its shaped can not be varied without sacrificing the efficiency
Reliable, high efficiency, Minimum of maintenance Like Shunt motors its speed decreases in increase of load
Its starting management is simple (squirrel motors) Its starting torque is someone inferior to DC Shunt motors
 Like Starting from rest & needs not extra starting motor
 Has not to be synchronized

3. Induction Motor: Construction
Consist of two main parts i) Stator ii) Rotator
STATOR: It is same as that of a synchronous motor & generator. It is made up of a number of stampings, which are slotted
to receive the windings. The stator carries a 3-phase winding & fed from 3-phase power supply. It is wound for definite
number of poles(P=2n where n is the number of stator slots/pole/phase), the exact number of poles being determined by the
requirement of speed. Poles inversely proportional to the speed.
When the stator winding is supplied by three-phase current , produce a magnetic flux, which is of constant magnitude
but which revolves at synchronous speed (given by Ns = 120 f/P). This revolving magnetic flux induces an e.m.f. in the rotator
by mutual induction
ROTOR:
Squirrel Cage Rotator: Motor employing this type of rotors are called Squirrel Cage Induction Motors
Phase Wound/ Wound Motor: Motors employing this type of rotors are variously known as “Phase wound” motors / “wound”
motors/ “Slip- Rings” motors

4. Induction Motor: Squirrel Cage Rotator
About 90% of induction motors are of this type, because this type of rotor has the simplest & most rugged construction
imaginable & is almost indestructible. The rotor consists of a cylindrical laminated core with parallel slots for carrying the rotor
conductors which, it should be noted clearly, are not wires but consists of heavy bars of copper, aluminum or alloys. One bar is
placed in each slots, rather the bars inserted from the end when semi-closed slots are used. The rotor bars are brazed or
electrically welded or bolted to two heavy & stout short-circuiting end rings, thus giving us, what is so picturesquely called, a
squirrel-case construction
It should be noted that the rotor bars are permanently short-circuited on themselves, hence it is not possible to add any
external resistance in series with rotor circuit for starting purposes
The rotor slots are usually not quite parallel tot the shaft but are properly given a slight skew. This is useful in two ways
i) It helps to make the motor run quietly by reducing the magnetic hum
ii) It helps in reducing the locking tendency of the rotor ie the tendency of the rotor teeth to remain under the stator teeth due to
direct magnetic attraction between the two
In small motors, another method of construction is used. It consists of placing the entire rotor core in a mould & casting
all the bars & end-rings in one piece. The metal commonly used is an aluminum alloy.
Another form of rotor consists of a solid cylinder of steel without any conductors or slots at all. The motor operation
depends upon the production of eddy currents in the steel rotor

6. Induction Motor: Phase wound Rotator
This type of rotor is provided with 3-phase, double layer, distributed winding consisting of coils as used in alternators. The
rotor is wound for as many poles as the number of stator poles & is always wound 3-phase even when the stator is wound
two-phase
The three phases are starred internally. The other three winding terminal are brought out & connected to three
insulated slip-rings mounted in the shaft with brushes resting on them. These three brushes are further connected externally to
a 3-phase star connected rheostat. This makes possible the introduction of additional resistance in the rotor circuit during the
starting torque of the motor & for changing its speed-torque/current characteristics. When running under normal conditions, the
slip-rings are automatically short-circuited by means of a metal collar, which is pushed along the shaft & connects all the rings
together. Next, the brushes are automatically lifted from the slip-rings to reduce the fractional losses & the wear & tear. Hence,
it is under normal running conditions, the wound rotor is short-circuited on itself just like the squirrel-case rotor
Shows the longitudinal section of a slip-ring motor, whose structural details are as under:
1. Frame: Made of close-grained alloy cast iron
2. Stator & Rotor Core: Built from high-quality low-loss silicon steel laminations & flash-enamelled on both sides
3. Stator & Rotor Windings: Have moisture proof tropical insulation embodying mica & high quality varnishes. Are carefully
spaced for most effective air circulation and are rigidly braced to withstand centrifugal forces & any short-circuit stresses

7. 4. Air-gaps: The stator rabbets & bore are machined carefully to ensure uniformity of air-gaps
5. Shaft & Bearings: Ball & Roller bearings are used to suit heavy duty, trouble-free running & for enhanced service life
6. Fans, Light aluminum fans: They are used for adequate circulation of cooling air & are securely keyed onto the rotor
shaft
7. Slip-rings & Slip-rings Enclosures: Slip rings are made of high quality phosphor-bronze & are of moulded construction

9. Induction Motor: Production of Rotating Field